1. Field of the Invention
The present invention relates to a shoe driving mechanism for a drum brake which, by controlling the pressing force of brake shoes against a drum, is able to ensure high braking performance and stability, is suitable for the adoption of an electrically powered system for an operating-force generating mechanism, and makes it possible to improve assembling efficiency.
2. Description of the Related Art
Conventionally, drum brakes of various types are used for braking running of vehicles. These drum brakes are classified into a leading trailing type, a two leading shoe type, a duo-servo type, and the like depending on the arrangement of the brake shoes which are pressed against an inner peripheral surface of a substantially hollow cylindrical drum.
Generally, the drum brake of the duo-servo type has a pair of brake shoes, including a primary shoe and a secondary shoe, which are disposed in face-to-face relation within a hollow cylindrical drum.
The primary shoe is arranged such that its inlet side in a forwardly rotating direction of the drum serves as an input portion, and its outlet side in the forwardly rotating direction of the drum is lined to an inlet side of the secondary shoe through, for example, an adjuster. Meanwhile, an outlet side of the secondary shoe abuts against an anchor portion mounted on a backing plate, and an anchor reacting force acting in the primary shoe and the secondary shoe is received by the anchor portion.
Consequently, if the primary shoe and the secondary shoe are extended and are pressed against the inner peripheral surface of the drum, the anchor reaction force acting in the primary shoe is inputted to the inlet side of the secondary shoe so as to act in such a way as to press the secondary shoe against the inner peripheral surface of the drum. Hence, both the primary shoe and the secondary shoe operate as leading shoes, so that it is possible to obtain a braking force with an extremely high gain.
As compared to the drum brakes of the leading trailing type and the two leading shoe type, the drum brake of the duo-servo type has numerous advantages in that not only is the apparatus able to obtain an extremely high braking force, but it can be easily downsized, and that it is possible to easily incorporate a parking brake.
However, since the above-described drum brake of the duo-servo type is sensitive to a change in the coefficient of friction of a brake shoe lining, the apparatus has a tendency that the braking force is difficult to stabilize. Hence, there has been a demand for providing a measure for stabilizing the braking force.
In view of the foregoing background, the applicant of this invention has already proposed a shoe driving mechanism in which a fluid-pressure controlling valve is incorporated in a wheel cylinder of a fluid pressure type for extending the pair of brake shoes, and the supply of fluid pressure to the wheel cylinder is controlled in correspondence with the anchor reaction force, thereby stabilizing the braking force.
However, with the braking apparatuses for vehicles in recent years, attempts to make brake functions intelligent are being actively undertaken, such as the provision of an antilock brake system and the provision of a traction control system. In addition, development of electric vehicles (EV) and hybrid vehicles is also being actively undertaken in view of the alleviation of environmental pollution and the like. To cope with the tendencies toward intelligent brake functions, electric vehicles, and the like, it has been an important issue to adopt an electrically powered system for the brake apparatus.
In adopting the electrically powered system for the brake apparatus, instead of the conventional wheel cylinder of the fluid pressure type, an operating-force generating mechanism of an electrically powered type making use of an electric motor or the like is adopted, for example. In that case, it becomes impossible to make use of the aforementioned shoe driving mechanism for controlling the supply of fluid pressure to the wheel cylinder of the fluid pressure type to a value corresponding to the anchor reaction force by means of a fluid-pressure controlling valve, and the development of a shoe driving mechanism adapted to the operating-force generating mechanism of the electrically powered type is newly required, which has been a new task in the adoption of the electrically powered system for the duo-servo type drum brake.
For this reason, as a shoe driving mechanism adapted to the operating-force generating mechanism of the electrically powered type, link mechanism have been proposed for transmitting the output of the operating-force generating mechanism to the pair of brake shoes. However, the shoe driving mechanisms based on the conventional link mechanisms have numerous component parts, and since the component parts must be assembled between the pair of brake shoes one by one, so that there has been a problem in that the operating efficiency in assembly is poor.
The present invention has been devised in view of the above-described circumstances, and its object is to provide a shoe driving mechanism for a drum brake which, by controlling the pressing force of brake shoes against a drum, is able to ensure high braking performance and satiability, which makes it possible to use any one of an actuator of an electrically powered or fluid-pressure type and a manual-type link mechanism and is therefore versatile, which has a relatively fewer number of component parts, and which makes it possible to improve the operating efficiency in assembly by assembling in advance the component parts as a unit and by subsequently assembling the unit between the pair of brake shoes.
To attain the above object, in accordance with the present invention, there is provided a shoe driving mechanism for a drum brake in which a shoe operating force generated by operating-force generating mechanism is transmitted to a primary shoe and a secondary shoe arranged in face-to-face relation in an inner space of a drum to press the brake shoes against the drum, and a pressing force of the brake shoes against the drum is controlled in correspondence with an anchor reaction force applied to an anchor pin, characterized by comprising: a base plate which is rotatably fitted to the anchor pin for the primary shoe; an anchor pin for the secondary shoe disposed uprightly at a position spaced apart a predetermined distance in the circumferential direction of the drum from an inserting position of the anchor pin for the primary shoe on the base plate; a first lever supporting pin disposed uprightly at a position spaced apart on a radially inward side of the drum from the inserting position of the anchor pin for the primary shoe on the base plate; a second lever supporting pin disposed uprightly at an intermediate position between the first lever supporting pin and the inserting position of the anchor pin for the primary shoe on the base plate; a balance lever whose proximal end is rotatably fitted to and supported by the first lever supporting pin, and in a distal end side of which is penetratingly formed a swinging-motion restricting hole through which the second lever supporting pin is passed and which restricts a range of its swinging motion about the first lever supporting pin, an outer periphery of the distal end side abutting against one end of the primary shoe; and an input lever which is brought into contact with the balance lever and the secondary shoe and is adapted to transmit the shoe operating force inputted from the operating-force generating mechanism to the balance lever and the secondary shoe, wherein the pressing force applied to the brake shoes is restricted by the behavior of the base plate and the respective levers corresponding to the anchor reaction force applied through the anchor pin for the secondary shoe and the input lever during braking.
The shoe driving mechanism for a drum brake according to the present invention is characterized in that after the anchor pin for the secondary shoe, the first lever supporting pin, the second lever supporting pin, the balance lever, and the input lever are assembled to the base plate as an assembly, the base plate is fitted to the anchor pin for the primary shoe so as to assemble the shoe driving mechanism to a backing plate of the drum brake.
Then, in accordance with the above-described arrangement, when the shoe operating force is inputted from the operating-force generating mechanism to the input lever of the shoe driving mechanism, the input lever swings so as to press the intermediate portion of the balance lever by the input transmitting portion, and press the secondary shoe toward the inner peripheral surface of the drum by a shoe abutting portion. The pressing force applied from the input lever to the balance lever due to the swinging behavior of the input lever causes the balance lever to swing toward the primary shoe side with the first lever supporting pin as the fulcrum of rotation, thereby pressing the primary shoe toward the inner peripheral surface of the drum. Then, the swinging motion of the balance lever toward the primary shoe side with the first lever supporting pin as the fulcrum of rotation terminates as the second lever supporting pin is brought into contact with one end of the swinging-motion restricting hole. Subsequently, the balance lever and the base plate swing as a unit about the anchor pin for the primary shoe, thereby pressing the primary shoe against the inner peripheral surface of the drum.
The pressing force applied from the input lever to the secondary shoe due to the swinging behavior of the input lever displaces the secondary shoe toward the inner peripheral surface side of the drum, thereby pressing the secondary shoe against the inner peripheral surface of the drum.
As described above, a braking force is generated as the shoe operating force is inputted from the operating-force generating mechanism to the input lever of the shoe driving mechanism, and the swinging motion of the input lever presses the respective brake shoes against the inner peripheral surface of the drum.
In a state inwhich the braking force is being generated, the anchor reaction force of the secondary shoe is applied to the anchor pin for the secondary shoe disposed uprightly on the base plate, and the anchor reaction force applied to the anchor pin for the secondary shoe causes torque to be applied to the base plate in a direction opposite to that of torque applied to the base plate by the shoe operating force.
Therefore, when the anchor reaction force reaches a predetermined magnification with respect to the shoe operating force, the torque applied to the base plate by the shoe operating force assumes a state of equilibrium with the torque applied to the base plate by the anchor reaction force. Then, when the anchor reaction force exceeds the predetermined magnification with respect to the shoe operating force, the torque applied to the base plate by the anchor reaction force overcomes the torque applied to the base plate by the shoe operating force. Consequently, the base plate swings toward the secondary shoe side with the anchor pin for the primary shoe as the fulcrum of rotation. This swinging motion of the base plate toward the secondary shoe side pushes back the input lever through the balance lever being retained by the first and second lever supporting pins, and applies to the input lever a braking limiting force in a direction of decreasing the action of the shoe operating force.